The effect of alloying, hot forging, and heat treatment on the microstructure, mechanical properties, and oxidation resistance of O-Ti2AlNb based intermetallic alloys

Abstract

In the present work, novel physicochemical approaches are proposed for the design of alloys based on the O(Ti 2 AlNb) phase by balancing Al and Nb equivalents to maintain a two-phase O+β/B2 composition, and alloying with elements having high melting points and/or larger atomic radii. The overall alloying strategy was tailored to critically enhance oxidation and creep resistance, while retaining acceptable ductility. Based on these approaches, four experimental multicomponent alloys were investigated and compared with the reference Ti-22Al-25Nb-0.15B alloy: Ti-22Al-21Nb-5.5(Zr,Mo,Ta,Sn,Hf)-0.3Si-0.15B, Ti-25Al-21Nb-5.5(Zr,Mo,Ta,Sn,Hf)-0.3Si-0.15B, Ti-23.5Al-23Nb-4(Zr,Mo,Ta,Hf)-0.3Si and Ti-23Al-25.5Nb-2.5(Zr,Mo)-0.3Si (in at.%). The alloys based on Ti-(22-23.5)Al consisted predominantly of O(Ti 2 AlNb) and β/B2 phases, whereas the Ti-25Al based alloy was nearly single-phase with only a minor amount of the β/B2 phase. DSC analysis showed an identical phase transformation sequence during continuous heating for all alloys: O(Ti 2 AlNb)+β/B2→β/B2+α 2 (Ti 3 Al)+O(Ti 2 AlNb)→β/B2+α 2 (Ti 3 Al)→β/B2→L. Long-term oxidation tests (700 °C/1000 h) revealed that complex alloying, higher Al content, and a decreased β/B2 fraction appreciably improved oxidation resistance. Tensile and creep behaviors were evaluated after isothermal 3D forging and heat treatment. Among the studied compositions, the Ti-23.5Al based alloy subjected to post-forging β/B2-annealing and aging exhibited the most balanced mechanical properties and improved oxidation resistance: UTS = 1071-1136 MPa and δ = 1.5-1.7% at room temperature, a creep rupture life of 141.5-174 h at 700 °C/340 MPa, and a low mass gain of about 0.8 mg/cm 2 after 1000 h of air exposure at 700 °C. A study of the microstructure-property relationships of this optimal composition highlights the advantages of the proposed multicomponent alloying strategy.